Power inverters are known in the art and used in many applications where it is necessary to generate alternating current power from a direct current power source. As just one example, inverters have been used to generate AC power for driving portable appliances, from the DC power available from a car cigarette lighter.
One particular application of an inverter relevant to the present patent application, is to power AC motors from a DC power source. Although this need arises in many environments, an environment of particular interest in this application is spacecraft, where the available power is typically DC power, available from a spacecraft power bus. A range of spaceborne sensors utilize cryocoolers based on the reverse-Brayton cycle. A single stage reverse-Brayton (SSRB) cryocooler provides 5 watts of cooling at 65 K for a range of space sensor applications. Key components for the cooler are a single-stage turboexpander, a high effectiveness heat exchanger, a single-stage motor-driven centrifugal compressor, and an electrical drive for the motor. Three-phase high frequency AC power is needed to drive the compressor motor.
Inverters/variable speed motor drives are available commercially from many suppliers, but unfortunately are not well suited to the described application. The highest frequency commercial inverters tend to have rectangular waveforms with high harmonic content. They also are not optimized for the low voltage-high current character of the compressor motor. Pulse width modulation (PWM) is used to eliminate higher harmonics in some advanced commercial inverters. PWM, however, relies on microprocessor control, and computation time generally limits output frequencies to 200 Hz and lower.
Furthermore, for spaceborne applications, it is important to maintain high efficiency. Harmonics in the inverter output should be avoided to whatever extent possible because they contribute losses in the motor without contributing useful output, lowering the efficiency of the motor.
A search of commercially available inverters failed to turn up anything suitable, establishing the need to develop a high frequency inverter specially tailored to the requirements of the SSRB compressor motor.